Electrographic formation of dye images

ABSTRACT

The present invention relates to an electrographic developer and method for the formation of water-soluble dye images. An electrostatic charge pattern is developed using a liquid developer having toner particles containing a polymeric material which is normally hydrophobic, but which is readily convertible to a hydrophilic form when contacted by an activating solution having a pH less 7.0. The toner particles have encapsulated therein a water-soluble dye of the desired color. The toner particle image is formed on a hydrophobic surface and is contacted with the activating solution to convert the toner particle image to hydrophilic form. The hydrophilic toner particle image is then contacted with a suitable receiving sheet whereupon the dye image migrates through the toner particle and is transferred to the receiver sheet.

United States Patent [191 Eastman et al.

[ Nov. 13, 1973 [75] Inventors: Donald R. Eastman; Stewart 11.

Merrill, both of Rochester, N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, N.Y.

[22] Filed: Sept. 22, 1972 [21] Appl. No.: 291,485

[52] US. Cl. 117/37 LE, 96/1 LY, 96/1.2, 101/464, 252/621 [51] Int. CL. G03g 13/00, 603g 13/10, G03g 13/14 [58] Field of Search 96/1 LY, 1.2; 117/37 LE; 101/464; 252/621 [56] References Cited UNITED STATES PATENTS 3,630,729 12/1971 Bach et a1 252/621 X 3,625,897 12/1971 Machida et al. 3,554,946 l/l971 Okuno et al. 252/621 Primary Examiner-Roland E. Martin, .Ir. Attorney-Robert W. Hampton et a1.

[5 7] ABSTRACT The present invention relates to an electrographic developer and method for the formation of water-soluble dye images. An electrostatic charge pattern is developed using a liquid developer having toner particles containing a polymeric material which is normally hydrophobic, but which is readily convertible to a hydrophilic form when contacted by an activating solution having a pH less 7.0. The toner particles have encapsulated therein a water-soluble dye of the desired color. The toner particle image is formed on a hydrophobic surface and is contacted with the activating solution to convert the toner particle image to hydrophilic form. The hydrophilic toner particle image is then contacted with a suitable receiving sheet whereupon the dye image migrates through the toner particle and is transferred to the receiver sheet.

11 Claims, No Drawings ELECTROGRAPHIC FORMATION OF DYE IMAGES This invention relates to the field of electrography and to methods of forming and transferring dye images using electrographic materials and techniques.

Electrographic imaging processes and techniques, especially electrophotographic processes, are wellknown in the art. These processes generally have in common the steps of forming an electrostatic latent charge image on an insulating surface and the development of this charge pattern to form the visible image by I the selective deposition thereon of finely divided electrostatically attractable toner or marking particles. Typically, these toner or marking particles contain a synthetic resin and, in addition, there may be associated with the resin a colorant material such as a pigment or dye. The visible image thus formed may be transferred to another support, such as a sheet of paper, or the visible image formed by the marking particles may be fixed to the insulating layer on which it is deposited to form a permanent image.

According to one form of electrography, the electrostatic charge pattern is made visible by the use of water-soluble dyes. The use of various water-soluble dyes to form visible electrographic images has been found particularly useful in the formation of multiple color electro graphic prints and transparencies. Various methods have been developed in the art for formation of dye images and the transfer of these images to various receiving sheets. In this regard, reference may be made to US. Pat. Nos. 3,003,891 issued Oct. 10, 1961, 3,060,052 issued Oct. 23, 1962, 2,843,499 issued issued July 15, 1958, and 3,253,913 issued May 31, 1966, and various foreign patent publications such as French Patents 1,369,344 published July 6, 1964, 1,245,790 published Oct. 3, 1960, and 83,756 (first addition to French 1,323,519) published Aug. 31, 1964.

In much of the work relating to the use of watersoluble dye image formation, a liquid development system is used for the formation of an image corresponding to the latent electrostatic charge pattern carried on an insulating substrate such as a photoconductive composition. However, various problems have been associated with the use of such liquid developers. Typically, these developers contain a synthetic resin having associated therewith the water-soluble dye as a colorant material. One problem relating to this type of liquid developer has been the fact that it may be difficult to find a colorant-binder combination having the proper hue and possessing the proper electrical charge characteristics for use in a liquid development system. Problems of this type are discussed, for example, in British Patent 1,016,581. In addition, although the colorant material may have the proper electrical charge characteristics and/or hue, it may be difficult to find a binder material suitable for use with such a colorant. For example, it has been found that various hydrophilic binder materials useful with such water-soluble dyes such as poly(vinyl alcohol), poly(vinylpyrrolidone), etc., such as is described in Chechak, U.S. application Ser. No. 58,191 filed July 24, 1970, now Def. Publ. T887,027, tend to become unstable, agglomerate and settle out of the developer soon after its preparation.

In addition to the above-described liquid development systems in which water-soluble dyes are used as colorants, still other types of liquid developers have been proposed in which water-soluble dyes may be used as colorant material. For example, British Patent 1,181,287 dated Feb. 11, 1970, apparently describes a liquid developer containing toner particles composed of a polymeric binder chemically bonded to various colorants including water-soluble dyes. Although such developers are apparently stable and may not exhibit substantial problems of toner-particle settling and agglomeration, these developers present substantial problems in the formation and transfer of dye images. That is, since the water-soluble dye colorants are chemically bonded to the polymeric binder component of the toner particles, the dye colorant cannot be readily separated from the polymeric binder. Accordingly, once such toner particles are deposited in imagewise fashion, there is no convenient way to separate the dye colorant from the polymeric binder so that the dye may be transferred to a suitable receiving element, thereby resulting in a dye image.

In accordance with the present invention there is provided an improved electrostatic liquid developer composition and electrographic method for the formation of water-soluble dye images and the transfer of such images to various receiving elements. According to the method of the invention, an electrostatic charge pattern carried on an electrically insulating surface is developed utilizing a liquid developer comprising a liquid carrier having admixed therein toner particles containing a soluble dye encapsulated in a binder which is normally hydrophobic in its free base form but which is readily convertible to a hydrophilic form when contacted by an acidic aqueous activating solution. In accordance with the invention, the developed tonerparticle image is formed directly on a hydrophobic surface or transferred to a receiving element having a hydrophobic surface. Subsequently, the developed image carried on the hydrophobic surface is contacted with the activating solution to form the hydrophilic form of the polymeric toner-particle image. As a result, any acidic soluble dye which is encapsulated in such polymeric toner particles can readily migrate through the water-swollen hydrophilic form of the polymer tonerparticle image and be transferred to a receiver sheet. In many instances, the receiver sheet may be prewetted with a suitable activating solution. In such case, conversion of the hydrophobic toner-particle image to its hydrophilic form and transfer of the water-soluble dye encapsulated in the toner particles to the receiver sheet will occur substantially simultaneously upon contact of the prewetted receiver sheet with the toner-particle image.

In any of the embodiments of the invention, it will be appreciated that, if desired, the hydrophobic tonerparticle image may itself be transferred to a suitable receiving element prior to use of the aforementioned activating solution. Such a mode of operation may be particularly advantageous where the electrostatic charge pattern to be developed is formed on a,reusable electrographic element, e.g., a reusable electrophotographic element containing a radiation-sensitive photoconductive layer.

As suggested hereinbefore, the process and developer of the present invention offers several significant advantages over many of the known developers and techniques used to form water-soluble dye images cor responding to an electrostatic charge pattern. For example, the electrostatic liquid developing compositions used in the present invention exhibit good stability; that is, the toner particles of the developer which contain the water-soluble dye colorants exhibit little tendency to agglomerate or settle. Such stability is apparently due, in part, to the high degree of compatibility between the hydrophobic free base form of the polymeric binder and the water-soluble dye colorants; thus, the colorants and binder may be readily admixed together to form the finely divided toner particles dispersed in the carrier vehicle of the developers of the present invention. Moreover, since the polymeric binder used in the toner particles of the invention may be readily converted to a hydrophilic form thereof, the water-soluble dye colorants encapsulated in such a binder may be readily separated from the binder and transferred to a suitable dye-receiving element, e.g., a mordantcontaining receiver paper, to form a binderless dye image thereon. In addition, since the liquid developer used in the process of the present invention contains toner particles composed of hydrophobic resins, rather than hydrophilic resins, the toner particles are compatible with conventional hydrocarbon carrier liquids, thereby further enhancing the stability and preventing substantial settling of the toner particles in the liquid carrier vehicle.

An important element of the present invention is the use of toner particles containing a polymeric material which is normally hydrophobic in its free base form, but which, when contacted by an activating solution of the appropriate pH, is readily convertible to a hydrophilic form of the polymer. In accordance with the invention, it has been discovered that polymers useful in the present invention are copolymers containing as a repeating unit thereof a group derived from an ethylenically unsaturated polymerizable monomer, said repeating unit having attached thereto a moiety capable of forming a salt when contacted with an activating solution of appropriate pH. Typical of such moieties are basic moieties such as nitrogen-containing moieties, e.g., amino styrene, vinyl pyridine, etc.

Polymers such as those described above may be formed by a number of well-known methods. For example, in accord with a first embodiment of the invention, a polymer formed from one or more ethylenically unsaturated monomers such as the soluble" and insoluble monomers described hereinafter may be prepared. Subsequently, substituents containing a basic moiety may be attached to the repeating units forming the backbone chain of the polymer. Useful basic moieties are described in greater detail hereinafter in connection with a preferred embodiment of the invention wherein the polymeric materials are formed from polymerizable monomers which initially contain a basic moiety. in accord with this first embodiment of the invention, the amount of basic moieties which should be incorporated in the resultant polymer to form a polymeric material capable of being converted to hydrophilic form when contacted by an activating solution of appropriate pl-I may vary. Typically, from about to about 50 percent by weight of the polymer should be composed of repeating units having a basic moiety attached thereto.

In accord with a preferred embodiment of the invention, the polymeric material comprising the toner particles is a polymerized blend of ethylenically unsaturated monomers, at least a portion of said monomers containing a moiety capable of forming a salt when contacted with an activating solution of appropriate pH. Typical of such monomers are polymerizable monomers containing basic moieties such as amino styrene, vinyl pyridine, etc.

Typical basic monomers useful in forming the polymer toner particles employed in the preferred embodiment of the present invention are basic monomers preferably comprising two to about l9 carbon atoms and having the following formula:

wherein R represents hydrogen; a halogen such as fluorine, bromine, chlorine or iodine; or an aliphatic alicyclic radical comprising one to eight carbon atoms; and wherein R- is a basic nitrogen-containing moiety typically comprising zero to about 15 carbon atoms. Particularly good results are obtained wherein R represents a nitrogen-containing moiety, including aliphatic, carbocyclic, and heterocyclic nitrogen-containing moieties comprising 0 to about 12 carbon atoms. Various useful such monomers include amino styrene, vinyl amine, vinyl pyridine, amino-substituted alkyl methacrylates and acrylates, vinyl imidazole, etc. In a preferred embodiment of the present invention, it has been found that vinyl pyridine provides an especially suitable such monomer because copolymers formed from such a monomer have been found to provide toner particles for a liquid developer which are extremely stable and resistant to agglomeration and settling. The amount of basic monomer useful in forming the copolymers employed in the toner particles of the present invention typically constitutes from about 10 to about 50 percent by weight of the monomeric blend (based on the dry weight of the blend absent any solvent or dispersion media) which is copolymerized to form the resultant copolymeric toner material.

It will be apparent to those skilled in the art that other basic monomers may be utilized in accordance with the present invention in addition to the various monomers noted specifically hereinabove. That is, by routine laboratory procedures one need only determine (1) whether a particular monomer under consideration is capable of copolymerizing to form a suitable copoly mer as described in detail hereinafter and (2) whether the monomer contains a moiety, such as a nitrogencontaining moiety, capable of forming a salt when contacted with an activating solution of suitable pH.

The remaining monomers used to form the copolymers employed in the toner particles of the present invention may be selected from a wide variety of wellknown polymerizable monomers useful in forming hydrophobic polymeric toner partciles for liquid developers. Representative of said monomers are polymerizable, ethylenically unsaturated materials such as styrene, e.g., halogenated, alkyl and alkoxy styrenes, alkyl acrylates and methacrylates, vinyl alkyl ethers, vinyl esters of aliphatic acids, etc.

Although the remaining monomers used to form the copolymers employed in the toner particles used in the invention may, as noted above, be selected from a variety of monomers, it has been found particularly useful, in accordance with a preferred embodiment of the invention, to utilize toner particles containing at least one copolymer prepared from a blend of monomers as described in detail hereinafter. These preferred copolymers have been found to provide toner particles which are extremely stable and exhibit good replenishment properties.

Such preferred copolymers are prepared from a monomeric blend and are generally characterized by a dispersibility ratio in the liquid carrier greater than about 0.825. As used in the present specification, the dispersibility ratio of a particular copolymer in a particular developer carrier liquid is defined by the following test. A 4.0-gram quantity of copolymer to be tested is admixed into 1 liter of a particular developer carrier liquid using a Waring or Polytron Blender operating within the range of 10,000 to 18,000 rpm. This mixture is then centrifuged at 34,000 G force for about 60 minutes. At the end of this time, the mixture is analyzed to determine the amount of polymer which has precipitated. To form the stable developers of the present invention, it has been determined that useful polymers should be dispersible to the extent that at least about 3.3 grams of the original 4.0-gram quantity of polymer remain suspended or dissolved in the carrier liquid after centrifuging. The dispersibility ratio is then calculated as the amount of polymer which remains suspended in the carrier liquid divided by the 4.0 grams of polymer oi'iginally mixed into the carrier liquid. A dispersibility ratio of 0.825 is equivalent to 3.3 divided by 4.0.

Exemplary of various additional monomers which may be utilized together with the basic monomers described hereinabove to form the above-noted preferred copolymeric toner particles for use in the present invention are soluble monomers, and insoluble monomers as set forth in Stahly and Merrill, U.S. application, Ser. No. 149,851 filed June 3, 1971, incorporated herein by reference thereto.

As used in the present specification, soluble monomeric moieties which can be copolymerized to form the copolymers used in the liquid developer of the invention are generally those moieties which, when polymerized, are capable of forming a homopolymer having an inherent viscosity as hereinafter defined of from about 0.4 to about 0.5 in chloroform at room temperature (about 25 C.) and a solubility (at 25 C.) in the carrier liquid to the extent that at least 5 parts by weight of polymer are soluble in 95 parts by weight of carrier liquid. In contrast,'the term insoluble" has reference to a monomeric moiety, a homopolymer of which, under the aforementioned viscosity conditions, is soluble in the carrier liquid to the extent of less than about 1 part by weight of polymer per 99 parts by weight of carrier liquid.

Representative soluble moieties which generally can be copolymerized to form the copolymers used in the liquid developers of the invention may be selected from the following group:

A. alkyl styrenes such as compounds having the formula:

wherein R is an alkyl having from about three to about 10 carbon atoms in the alkyl moiety;

B. alkoxy styrenes such as compounds having the formula:

wherein R is an alkyl having from about three to about 10 carbon atoms in the alkyl moiety, for example, pamyloxystyrene;

C. alkyl acrylates such as compounds having the formula:

wherein R is an alkyl having from about eight to about 22 carbon atoms in the alkyl moiety;

D. alkyl methacrylates such as compounds having the formula:

wherein R is an alkyl having from about eight to about 22 carbon atoms in the alkyl moiety;

E. vinyl alkyl ethers such as compounds having the formula:

wherein R is an alkyl having from about eight to about 22 carbon atoms in the alkyl moiety; and

F. vinyl esters of aliphatic acids such as compounds having the formula:

wherein R is an alkyl having from about six to about 22 carbon atoms in the alkyl moiety;

and mixtures thereof. Other soluble moieties may also be used.

Preferred soluble moieties which may be used in preparing the copolymers contained in the liquid developers used in the invention generally include the followmg:

A. alkyl styrenes having from about five to about 10 carbon atoms in the alkyl moiety;

B. alkyl acrylates and methacrylates having from about 12 to about 22 carbon atoms in-the alkyl moiety; and

C. vinyl esters of aliphatic acids having from about 10 to about 22 carbon atoms in the alkyl moiety;

and mixtures thereof. Typical soluble moieties or groups which can be so used include the following:

4-pentyl styrene 4-hexyl styrene 4-octyl styrene lauryl acrylate hexadecyl methacrylate octadecyl methacrylate eicosyl acrylate docosyl methacrylate vinyl caprate vinyl laurate vinyl palmitate vinyl stearate vinyl eicosate vinyl docosate and mixtures thereof.

The term inherent viscosity," as used herein, is defined by the following formula:

1;, In (1 solution/1; solvent) /C wherein 1 solution viscosity the voscosity of the solution, 1; solvent is the viscosity of the solvent and C is the concentration in grams per 100 ml. of the polymer solvent. The determination is made at a concentration of 0.25 gram of polymer in 100 ml. of chloroform at a temperature of 25 C.

As noted, the preferred copolymers used in the preparation of the liquid developers of the invention may also contain, if desired, at least one insoluble monomer copolymerized with the aforementioned soluble monomers. Representative insoluble monomers which may be suitable for being so copolymerized include the following:

A. styrenes selected from the group of styrene, methylstyrene, methoxystyrene and a halogenated styrene;

B. alkyl acrylates having from about one to about four carbon atoms in the alkyl moiety;

C. alkyl methacrylates having from one to about four carbon atoms in the alkyl moiety;

D. vinyl alkyl ethers having from one to about four carbon atoms in the alkyl moiety; and

e. vinyl esters of aliphatic acids having from about one to about four carbon atoms in the alkyl moiety, and mixtures thereof.

Preferred insoluble monomers which may be used in preparing the copolymers contained in the developers used in the present invention generally include the following:

A. styrene and methylstyrene;

B. alkyl acrylates having from one to about four carbon atoms in the alkyl moiety;

C. alkyl methacrylates having from one to about four carbon atoms in the alkyl moiety; and

D. vinyl esters of aliphatic acids having from one to about four carbon atoms in the alkyl moiety;

and mixtures thereof. Typical insoluble moieties or groups which can be so used include the following:

styrene a-methylstyrene ethyl acrylate methyl acrylate butyl acrylate ethyl methacrylate propyl methacrylate butyl methacrylate vinyl acetate vinyl propionate vinyl butyrate and mixtures thereof.

It will be apparent that the choice of particular soluble or insoluble moieties is determined by a number of factors. The degree of solubility in the carrier liquid may be controlled by proper adjustment of the ratio of soluble moiety to insoluble moiety. In addition, the nature of the particular soluble monomeric moiety, such as the degree of solubility of a homopolymer comprising it, will influence the particular insoluble monomeric moiety chosen to copolymerize with it to give the final copolymer. For example, if the soluble monomer is one having a relatively long alkyl group attached to it, rendering a polymer containing it relatively soluble, the insoluble monomer is desirably one having a relatively short alkyl group attached to it, to balance the properties. On the other hand, a relatively short alkyl group on the soluble monomer in general requires a somewhat longer alkyl group on the insoluble monomer. Generally, as indicated above, especially preferred polymers useful in the present invention are dispersible in the carrier liquid to the extent that, if a 4.0-gram quantity of polymer is added to 1 liter of carrier, at least about 3.3 grams will remain dispersed therein after centrifuging the mixture at 34,000 G force for about minutes.

In general, the copolymers comprising the liquid developers of the invention may be prepared by an addition polymerization reaction wherein all of the component monomers are combined in a reaction vessel in a reaction medium, such as dioxane, and a suitable free radical initiator. The vessel containing the solution is then flushed with an inert gas, such as nitrogen, and heated to a temperature sufficient for the polymerization reaction to proceed at a reasonable rate. The temperature, in general, is above room temperature and preferably about 40 to 100 C. After the copolymer has formed, it is removed from the reaction mixture and purified as necessary. Copolymers produced according to this procedure typically have an inherent viscosity, as hereinbefore defined and measured, in the range of from about 0.1 to about 0.8. The preferred copolymers typically contain (a) one or more different recurring units derived from soluble monomers, said units containing no basic side-chain moiety and (b) one or more different recurring units containing as a sidechain substituent a basic moiety. If desired, the copolymer may also contain as a recurring unit thereof one or more different units derived from insoluble monomers. In general, a typical preferred copolymer used in the liquid developers of the invention may contain from about 10 to about 70 weight percent of recurring units derived from soluble moieties, said units containing no basic side-chain moiety, and from about 0 to about weight percent of recurring units derived from insoluble moieties. The dispersibility of the polymer can be adjusted as desired by proper balancing of the relative abundance of the soluble and insoluble monomers. Mechanical properties such as abrasion resistance and fixability of the resultant toner image can also be adjusted by properly balancing the ratio of the components in the polymer.

It should be noted that the relative abundance of the various starting monomers in the polymerization medium is not always indicative of the percentage composition to be expected in the resultant polymer. When acrylic monomers are used as starting material, for example, it is found that the composition of the polymer bears a very close correlation to the relative abundance in the starting solution, whereas when styrenes are used as starting materials, the correlation is not so close. Such deviations are well-known to the polymer chemist, and one skilled in the art should readily be able to produce such variations in composition as may be desired to meet particular requirements.

Liquid developers containing the polymers described herein typically comprise a dispersion of the polymer in a suitable carrier liquid. A common method of preparing such a dispersion is solvent-milling. A quantity of the polymer is dissolved in a suitable solvent and the solution placed in a ball mill. Water-soluble dye colorants and other additives, e.g., suitable charge control agents, which may be necessary or desirable, are added to the mix and the whole milled for a suitable time, typically up to a week. Alternatively, a viscous solution of the polymer is placed on compounding rolls having chilled to C.) water passing through the cooling system. Other additives are then placed on the rolls and thoroughly mixed and blended with the polymer. The dye colorant is generally present in an amount of from about 10 percent to about 60 percent of the weight of the resin. After passing the complete mix through the mill several times to blend the ingredients completely, the mix is removed.

Liquid developers are made from the toner concentrate formed as above by dispersing the concentrate in a suitable electrically insulating carrier liquid. Carrier liquids which may be used to form such developers can be selected from a wide variety of materials. Preferably, the liquid has a low dielectric constant and a fairly high electrical resistance such that it will not disturb or destroy the electrostatic charge pattern being developed. ln general, useful carrier liquids should have a dielectric constant of less than about 3, should have a volume resistivity greater than about 10 ohm-cm, preferably greater than about 10 ohm-cm, and should be stable under a variety of conditions. Suitable carrier liquids include halogenated hydrocarbon solvents, for example, fluorinated lower alkanes, such as trichloromonofluoromethane, trichlorotrifluoroethane, etc., having a boiling range typically from about 2 C. to about 55 C. Other hydrocarbon solvents are useful, such as isoparaffinic hydrocarbons having a boiling range of from about 145 C. to about 185C., e.g., lsopar G (Humble Oil and Refining Co.), or cyclohydrocarbons such as cyclohexane. Additional carrier liquids which may be useful in certain situations include polysiloxanes, odorless mineral spirits, octane, etc.

As described hereinabove, the activating solutions utilized in the process of the present invention are selected to have a pH appropriate to convert the basic organic moieties contained in the copolymeric toner particles described hereinabove to a hydrophilic form. That is, since the toner powder particles utilized in the present invention contain a basic organic moiety capable of being converted to its hydrophilic form, the activating solution selected should have an acidic pH, i.e., a pH less than 7.0. Typically, the activating solution comprises a mixture of an acid in a suitable polar solvent such as an aqueous medium, e.g., water, wateralcohol mixtures, etc.

Concerning an acid activating solution, the relative proportion of acid and solvent may vary within a relatively wide range. Typically, the acid activating solutions have a pH less than about 6.5, preferably below about 5.0. The particular acid utilized to acidify the activating solution is not critical. The particular acid selected typically is chosen on the basis of its compatibility with other addenda which may be present in the system or solution, for example, water-soluble dyes which are incorporated in the polymeric toner particles. Naturally, the acid selected should not have a deleterious effect on the toner-particle image contacted by the activating solution. Typical acids which may be employed include the following: acetic acid, formic acid, propionic acid, chloroacetic acid, acrylic acid, glycolic acid, fumaric acid, oxalic acid, phosphoric acid, hydrochloric acid, sulfuric acid, potassium, bisulfate, and mixtures thereof. An activating solution which has been found to work well and is compatible with the components utilized in a typical electrostatic toner-particle image-bearing element and conventional water-soluble dyes is an aqueous acetic acid solution.

An acidic water-soluble dye, as used herein, is a water-soluble dye having as substituents at least one acid radical selected from sulfonic acid or carboxy acid radicals which are capable of forming salts with alkaline metals commonly used in the dye trade, such as sodium, potassium, lithium, etc. This term also includes the salts of such dyes, for example, the sodium, potassium, lithium, ammonium, triphenylguanidine, diphenylguanidine, di-o-tolylguanidine, aminoguanidine, cobalt, zinc, cadmium, copper, calcium and barium salts. Other salts in addition to those noted above could also be used.

The choice of useful dyes to be employed in the process of the present invention is generally determined by their solubility and spectral absorption characteristics. As will be apparent, the dyes utilized may be chosen from a wide variety of known dye classes. For example, dyes selected from the following classes are useful: nitro, azo including disazo, etc., arylmethane including diand tri-aryl methane, methine, anthrapyridones, indigo-type dyes, azines, thiazines, oxazines, acridines, etc. These and other useful classes of dyes can be found in the preamble to the Colour Index, Vol. 3, Second Edition, 6. From the standpoint of color, solubility and ease of transfer, the following are among the preferred water-soluble acid dyes: Brilliant Alizarine Light Red B (C.l. Acid Red 58), Erio Fast Cyanine S (CI. 63010), Tartrazine (C.l. 19140), Alizarine Astrol B (C.I. 61530), Alizarine Rubinol R (CI. 68215), Alizarine Viridine FF (CI. 62555), Fast Light Yellow 30 (C.l. 19120), copper phthalocyanine tetrasulfonic acid sodium salt (C.l. 74220), and the dyes exemplified by the following structures:

SOaNa.

wherein R is an alkyl radical of from 1 to 12 carbon atoms with l to 7 carbon atoms being preferred or a phenyl radical, including substituted phenyl such as tolyl, xylyl, etc.

atoms with l to 4 carbon atoms being preferred.

(Flla soars-.1 5. I

(I l-CH3 NH on OCH: OH NHz l NaOasx/v $0 M 0 CH3 NaOgS Compounds having the structural formulas 1 through 6 can be named as follows:

1. 4,4'-bis( l-acylamino-3,6-disodiumsulfo-8- hydroxynaphthalene-7 -azo )-2 ,2 ',5 ,5 '-tetramethyltriphenylmethane;

2. 4,4'-bis(p-alkoxyphenylazo)-2,2'-disodiumsulfostilbene;

3. 4,4'-(6,8-disodiumsulfo-2- naphthylaminocarbonyl-p-phen-ylazo )-3 ,3 dimethylurea;

4. 1-(p-sulfophenyl-3-methyl-4-(4,8-disulfonaphthyl- 2-azo)-5-pyrazolone trisodium salt;

5. (l-hydroxy-3 ,6-disodiumsulfo-8- acetylaminonaphthyl-2-azo)-4-( 1-hydroxy-3 ,6- disodium sulfo-8-am inonaphthyl- 2-azo )-2 ,5 dimethoxy benzene;

6. l-(p-sulfophenyl-3 -phenyl-4-( 2,5 -d ichloro-4- sulfophenyl-l-azo)-5-pyrazolone disodium salt.

Other acidic dyes which are illustrative of the many useful dyes include the following:

7. l-carboethoxy-3-methyl-6-( 4'-chloro- 2 sulfoanilino)-2-oxo-2,3-dihydrobenzanthrone, sodium salt;

8. 5 ,5 '-ureylenebis[ 2(2-amino-6-sulfo-8-hydroxyl naph-thylazo)benzenesulfonic acid], tetrasodium salt, CI. 25380;

9) 3,3'-[4,4'-ureylenebis( 2- methylphenylazo)]bis( l ,S-benzenedisulfonic acid), tetrasodium salt, C.I. 29025;

5-{2anilino-4-[4-(3-carboxy-4 hydr0xyphenylazo)anilino]-s-triazin-2-ylamino }-4-hydroxy-3-{4-(8-hydroxy-3 ,o-disulfo-l naphthylazo)-2-rnethoxy-3-methylphenyl-azol- 2,7-naphthalenedisulfonic acid, pentasodium salt, C.l. 34045;

1 l. 6,6-ureylenebis 2- [4-(2 ,4dichloro-6-sulfophen ylazo)3 methylphenylazo]- l -naphthol-3 -sulfonic acid}, tetrasodium salt, CI. 35785; and

12. anhydr0-3,6-bis(diethylamino )-9-(2,4-disulfophenyl)-xanthylium hydroxide, sodium salt, C.l. 45100.

\\ O NS.

Dyes such as 7-12 are sold under various names, including the following:

Solantine Pink 48L, C.l. 25380 Solantine Yellow RL, CI. 29025 Pontamine Fast Green G, C.l, 34045 Sirius Rubine 6B, CI. 35785 Xylene Red B, C]. 45100 Solophenyl Fast Blue Green BL, Cl. Direct Green 27 Although, as noted hereinabove, a wide range of water-soluble dyes may be employed in the process of the present invention, best results have typically been obtained utilizing acid dyes. The mordantable acid dyes especially have been found highly useful because of their mordantability which permits dye images formed from these dyes to be easily superimposed one on top of the other to form sharp, multiple dye images on a single dye-receiving sheet. The mordantability of these dyes is especially useful in such a multiple-image superimposition process as it prevents the tendency of a previously formed dye image from deteriorating due to diffusion or wandering of the dyes which tends to occur during a subsequent dye-transfer operation. Suitable mordantable dyes include those dyes often referred to in the dye trade as dye-transfer dyes."

The electrostatic charge image utilized in the process of the present invention may be formed by a variety of well-known techniques. For example, a typical electrographic electrostatic image may be formed employing any of a number of well-known photoconductive materials including inorganic photoconductors, organic photoconductors and organometallic photoconductors. A typical electrographic process, as is well-known, involves uniform charging of the photoconductive element followed by subsequent imagewise exposure, thereby forming an electrostatic charge pattern corresponding to the imagewise exposure. Such a process can be developed to form either positive or negative images of the original exposed image. This may be done simply by the appropriate use of development electrodes in the development process, all of which is wellknown in the electrographic art. Various other techniques for forming the electrostatic latent image may be also used, such as persistent conductivity image formation wherein a photoconductive surface is exposed to an optical image to form a latent conductivity image which is then subjected to an electrical charge using, for example, a corona charge device. In such a process the latent conductivity areas, being conductive, will dissipate the surface charge in the previously exposed areas, whereas in the unexposed areas the surface charge will remain to form an electrostatic image which is a reversal of the latent conductivity image. Other electrostatic imaging techniques which may be used include electrostatic printing or recording by the use of imagewise-charging electrodes. Further detail concerning persistent conductivity imaging process and electrographic printing processes may be found, for example, in the book entitled Electrophotography by R. M. Schaffert, chapters IV and IX, respectively, published by The Focal Press in 1965. A variety of other particularized methods for forming electrostatic patterns using electrographic techniques may also be found in the Schaffert text noted above.

Having formed the electrostatic charge pattern, the next step utilized in the present invention involves the development of that charge pattern using the liquid developer containing the polymeric toner materials described in detail hereinabove. Following the application of the liquid developer to the electrostatic chargebearing element, there is formed a toner-particle image corresponding to the electrostatic charge pattern As explained hereinabove, depending upon whether a development electrode or other reversal technique is utilized, the toner-particle pattern thus formed may be either a positive or a negative image of the original image. The electrostatic charge pattern which is carried on an electrically insulating substrate may be liquiddeveloped by contacting the charge pattern with the liquid developers described hereinabove. This may be accomplished by a variety of techniques such as rolling the liquid developer over the charge pattern-bearing element, dipping the charge pattern-bearing element into a tank or trough containing the liquid developer, spraying the liquid developer onto the electrostatic charge pattern-bearing element, etc.

Upon development of the electrostatic charge pattern to form a polymeric toner-particle image corresponding thereto, the toner-particle image pattern may be directly subjected to the above-described activating solution. Or, in accordance with the other embodiments of the invention, the polymeric toner-particle image pattern may first be transferred to a receiver element and then subjected to the activating solution. This pattern. In such case, it may be desirable to transfer the tonerparticle image as soon as possible so that the charge-bearing element may be recycled for use in a subsequent electrostatic imaging operation. In addition, it may be desirable to avoid subjecting the element used for formation of the electrostatic charge pattern to the subsequent activating operation to avoid contaminating this element with such materials.

Methods of transferring liquid-developed tonerparticle images from the surface of one element, e.g., electro-photographic elements having a photoconductive layer, to a receiving element are well-known in the electrographic art. The use of a particular method or methods does not constitute a critical element of the present invention. Accordingly, an extended discussion thereof is deemed unnecessary in the present specification. Generally, transfer of liquid-developed images can be accomplished by impression in much the same manner as ink images are transferred from a printing plate. Electrostatic transfer may also be used. In general, except as noted herein or as required by a particular transfer operation, no special coatings are required on the surface of the receiving element.

Regardless of whether the toner-particle image is subjected to the activating solution on the original electrographic element utilized for formation of the electrostatic charge pattern or whether the toner-particle image is transferred from such an element to a subsequent receiver element, it is important that the tonerparticle image be carried on a substantially hydrophobic-surface when contacted with the activating solution. This insures that good dye image differential is ultimately obtained. Moreover, it will be apparent that the particular hydrophobic surface chosen should further be characterized as a hydrophobic material which is chemically inert with respect to the activating solution; that is, the hydrophobic surface should be composed of a material or materials which are not converted to a hydrophilic substance when contacted with the activating solutions.

The terms hydrophilic and hydrophobic as used throughout the present specification and claims are used in accordance with their conventional meanings. Thus, a hydrophobic material is a material lacking an affinity for water, whereas a hydrophilic material is a material having a strong affinity for water, i.e., water is readily absorbed or adsorbed thereto. As is apparent,

therefore, the hydrophobic surface on which the copolymeric toner particles of the present invention are carried may be composed of a variety of various substances including a large number of organic materials including synthetic and natural organic materials such as various polymers, rosins, waxes, etc., to impart hydrophobicity to the surface. A partial listing of suitable hydrophobic materials includes polystyrene, poly(- methyl methacrylate), polyethylene, bisphenol A, polycarbonates, polyacrylamides, polyamides, unhydrolyzed cellulose acetate, cellulose acetate butyrate, poly(vinyl acetate), poly(vinyl chloride), poly(ethylene terephthalate), silicone resins, poly(vinyl butyral), cellulose nitrate, cumar resins, shellac, and alkyds.

Having contacted the toner-particle image carried on a hydrophobic receiving surface with the activating solution, the resultant dye image may then be transferred to another receiving sheet, for example, a mordantcontaining receiving sheet, to form a more permanent dye image. As noted hereinbefore, contact of the tonerparticle image with the activating solution and transfer of the dye encapsulated in the toner particle to the receiving sheet may occur substantially simultaneously by prewetting the receiving sheet with the activating solution.

A suitable mordant contained in the receiving sheet is especially useful where one wishes to obtain a more stable permanent dye image. Transfer is typically accomplished by intimately contacting the activated dye toner-particle image with the receiving surface. The mordant, of course, reacts with the dye of the tonerparticle image to form a stable, fixed dye-mordant image on the receiving surface. A variety of wellknown mordanting compounds may be used in this embodiment of the invention. Such mordants are thoroughly familiar to those acquainted with the use of water-soluble acid dyes,

The particular mordants selected are not critical and generally are selected on the basis of their utility with the particular acid water-soluble dyes used in the process of the present invention. For example, if acid dyes are to be mordanted, the image-receiving surface used in this embodiment of the invention can contain basic mordants such as polymeric aminoguanidine derivatives of vinyl methyl ketone such as described in U.S. Pat. No. 2,882,156 by Minsk issued Apr. 14, 1959, and basic polymeric mordants such as described in copending U.S. application Ser. No. 100,491 of Cohen et al. filed Dec. 21, 1970, now U.S. Pat. No. 3,709,690. Other mordants useful in the invention include poly(4- vinylpyridine), the poly( l-methyl-2-vinylpyridinium p-toluene sulfonate) and similar compounds described in U.S. Pat. No. 2,484,430 by Sprague et al issued Oct. 11, 1949, and cetyltrimethylammonium bromide, etc. Effective mordanting compositions are also described in U.S. Pat. Nos. 3,271,148 by Whitmore and 3,271,147 by Bush. The mordanting compositions described in the Whitmore patent comprise at least one hydrophilic organic colloid containing a finely divided, uniform dispersion of droplets or globules of a highboiling, water-immiscible organic solvent in which is dissolved a high concentration of a cationic, nonpolymeric, organic dye-mordanting compound for acid dyes. The mordanting compositions described in the Bush patent comprise at least one hydrophilic organic colloid containing a finely divided, uniform dispersion of particles of a salt of an organic acidic composition containing free acid moieties and a cationic, nonpolymeric, organic dye-mordanting compound for acid dyes. Useful cationic or basic organic dye-mordanting compounds for dyes include quaternary ammonium and phosphonium, and ternary sulfonium compounds in which there is linked to the N, P or S onium atom at least one hydrophobic ballast group such as longchain alkyl or substituted alkyl groups.

Typically, the dye image-receiving surface contains a hydrophilic, organic colloid layer such as N- methoxymethyl poly-(hexamethylene adiparnide); partially hydrolyzed poly(vinyl acetate); poly(vinyl alcohol) with or without plasticizers; cellulose acetate; gelatin; and other materials of a similar nature. Furthermore, depending on the particular dye, the dye image receiving surface may be sufficient by itself to mordant the dye as is the case with the aforementioned colloids which are alkaline solutionpermeable layers. Generally, good results are obtained when the dye image receiving surface is a layer of about 0.25 to about 0.04 mil in thickness. This thickness, of course, can be modified depending upon the result desired. The dye imagereceiving surface can also contain ultraviolet-absorbing materials to protect the dye images from fading due to ultraviolet light and/or brightening agents such as the stilbenes, coumarins, triazines, oxazoles, etc.

As suggested hereinbefore, the present invention is especially useful in providing multiple-color copies of an Original image pattern. It will be understood, of course, that the invention has application in any embodiment wherein it is desired to form a water-soluble dye image of an original image pattern using an electrographic imaging system. However, as will be apparent to those skilled in the art, the invention has particular applicability to multiple-color electrographic systems wherein it is desired to form color copies of an original image pattern composed of multiple colors. As an example of a typical color system, a multiple-colored composite dye image of an original image pattern may be prepared in accord with the present invention as follows: A series of color separation positive images corresponding to the original image pattern is prepared. Preparation of such color separation images is wellknown in the photographic and electrographic arts and extended discussion thereof is deemed unnecessary. ln general, such color separation images are formed by preparing a separate color copy of the original image pattern corresponding to each of the primary colors, i.e., red, blue and green. If desired, a separate color separation image may also be made corresponding to the black color of the original image pattern. The resultant color separation images may then be used to form individual electrostatic charge images, one charge image corresponding to each color separation image, on separate electrographic elements, e.g., organic photoconductor-containing elements. The resultant charge images, as set forth hereinabove, are formed on a hydrophobic surface.

Having thus formed an electrostatic charge image on an electrographic element having a hydrophobic surface (one charge image for each of the individual color separation images), liquid development of these charge images may be performed. In a typical multiple-color processing system, each electrostatic charge separation image is developed with a liquid developer of the present invention containing, as a colorant, a water-soluble dye having a color complementary to the original color separation image. Thus, the electrographic element containing the charge pattern corresponding to the red color separation image is liquid-developed with a liquid developer containing a cyan dye as colorant; the electrographic element containing an electrostatic charge image corresponding to the blue separation image is developed with a liquid developer containing a yellow dye as colorant; and the electrographic element containing the electrostatic charge pattern corresponding to the green color separation image is liquid-developed with the developer containing a magenta dye as colorant. Subsequently, each of the resultant liquiddeveloped electrographic elements may be reacted with an aqueous acidic activating solution as described hereinabove to convert the hydrophobic polymeric binder contained in each of the toner-particle images to its hydrophilic form. Then, each of the resultant toner-particle images may be contacted one at a time in register to a receiving element having a suitable receiving surface such as a hydrophilic organic colloid as described hereinabove. Accordingly, the dye images from each of the individual toner-particle images are transferred in register and superimposed one on top of the other on the surface of the receiving element to form the final multiple-color copy of the original image pattern. Preferably, as described hereinabove, the surface of the receiving element contains a mordant for the dye images to reduce any tendency for the individual dye images to migrate as they are transferred and superimposed one on top of the other. Also, as discussed hereinabove, the process may be simplified byprewetting the receiving surface of the receiving element with the aqueous acidic activating solution so that the conversion of the hydrophobic toner particles to hydrophilic form may occur substantially simultaneously with the transfer of the dye images from the toner-particle images to the surface of the receiving element.

The following examples are presented to illustrate further the invention.

EXAMPLE 1 Preparation of polymeric binder for toner particles composed of poly(2-methyl-5-vinylpyridine-co-methyl methacrylate-co-lauryl methacrylate) A solution of g. 2-methyl-5-vinylpyridine, 22.5 g. of methyl methacrylate, 12.5 g. of lauryl methacrylate, 0.5 g. of azobisisobutyronitrile in 100 ml. of dioxane is flushed with nitrogen and placed in a 70 C. water bath for hours. The polymer solution, after dilution with 50 ml. of dioxane, is poured slowly into water in a home blender. The precipitated polymer is recovered by filtration, washed, air-dried, and finally dried in vacuum at 50 C.

This type of polymer functions well in the dyetransfer system of the invention because it is converted from the hydrophobic, free base form to the hydrophilic, salt form by the acid activating solution described previously herein. The dye can then diffuse through the water-swollen polymer and be transferred to a receiving sheet.

EXAMPLE 2 Formation of liquid developer The formulation of the liquid developer of the invention is accomplished using the following materials:

The polymer is dissolved in the Solvesso 100 and then poured into a Waring Blender with a Polytron head. The cobalt naphthenate solution is then added. The dye is then added slowly to this solution while running the blender at high speed for several minutes. A developer concentrate is thus formed.

into 500 ml. of lsopar G, 50 m1. of the above concentrate is poured slowly and blended in the Polytron for several minutes, resulting in a liquid developer composed of finely divided polymer toner particles (having dye encapsulated therein) dispersed in the lsopar G liquid-carrier vehicle (lsopar G is a trademark of Humble Oil and Regining Co. used to identify a liquid mixture of isoparaffinic hydrocarbons having a boiling point in the range of C. to C.).

EXAMPLE 3 An electrographic element having an electrically insulating organic photoconductive layer coated on a conductive support is charged to a negative surface potential of 700 volts, then contact-exposed to a positive, half-tone transparency illuminated by 3,000 K radiation. The resulting electrostatic charge pattern is then developed for several seconds in the liquid developer of Example 2. After development, the surface of the photoconductor element is rinsed briefly with lsopar G to remove developer solution and dried with warm air from a hair dryer. The photoconductor element, now bearing an imagewise deposit of dye-containing toner particles is then rolled into contact with a receiving element comprising a paper base having a gelatin overcoat containing a basic mordant (this element is commercially available under the trademark of Kodak Dye Transfer Paper). The gelatin surface layer of the element has previously been conditioned for one-half hour in an aqueous activating solution containing acetic acid (this solution is commercially available under the trademark of Kodak Conditioner Solution). Prior to transfer, excess activating solution is removed with a rubber squeegee. During the 60 seconds contact time, the dye transfers to the receiver paper, but the polymer remains on the photoconductor surface. This polymer image can be readily removed from the photoconductor surface by swabbing the surface with a cotton pad moistened with lsopar G.

EXAMPLE 4 To illustrate the continuous-tone capabilities of the system, another print is made in the same manner as described in Example 3 except that the half-tone transparency is replaced with a 0.2-increment photographic step tablet. Again, the dye encapsulated by the polymer toner particles is transferred to the receiving element.

As will be appreciated many variations and extensions of the invention described above may be effected. For example, multiple water soluble dye image copies may be made by proper control of the transfer operation wherein the receiver sheet is contacted with the activated water-soluble dye containing toner particle image. That is, by suitably adjusting the pressure exerted by the receiver sheet against the dye-toner particle image, the amount of dye transferred to a particular receiver sheet may be regulated so that it is possible to form multiple dyed receiver sheet copies of a single dye-toner particle image.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. In a method of liquid developing an electrostatic charge pattern carried on an electrically insulating substrate to obtain a water-soluble dye image corresponding to the charge pattern, the improvement which comprises:

A. forming on a hydrophobic surface an image of electrostatically attractable toner particles corresponding to such charge pattern using a liquid developer comprising said toner particles admixed in a liquid carrier vehicle having a dielectric constant less than about 3.0 and a resistivity greater than about ohm-cm, said toner particles comprising a binder and a colorant consisting essentially of an acidic, water-soluble dye, said binder comprising a hydrophobic copolymer having as a repeating unit thereof a group derived from an ethylenically unsaturated polymerizable monomer, said unit having attached thereto a basic nitrogen-containing moiy;

B. contacting said toner particle image carried on said hydrophobic surface with an activating solution comprising an acidic aqueous solution having a pH less than about 7.0 to convert said hydrophobic copolymer contained in said toner particles to the hydrophilic form thereof; and

C. contacting said hydrophilic dye-containing toner particle image carried on said hydrophobic surface with a receiving element to transfer the dye image to said receiving element.

2. The invention of claim 1 wherein said receiving surface comprises a hydrophilic colloid layer contain ing mordant for said acidic water-soluble dye.

3. The invention of claim 1 wherein (B) and (C) are performed substantially simultaneously by pre-wetting the receiving element with said activating solution and contacting the resultant wetted receiving element with the toner particle image formed in (A).

4. The invention as described in claim 1 wherein said ethylenically unsaturated polymerizable monomers comprises two to about 19 carbon atoms and a basic nitrogen-containing moiety.

5. The invention as described in claim 1 wherein said ethylenically unsaturated polymerizable monomer is selected from the group consisting of aminostyrene, vinyl pyridine, vinyl amine, amino-substituted alkyl methacrylates, amino-substituted alkyl acrylates and vinyl imidazole.

6. The invention of claim 1 wherein a multiple-color image is obtained on said receiving element by successively repeating (A) to (C) to form superimposed, in register, on said receiving element a series of individual dye images, each of said images representing an individual color separation image of an original image.

7. In a method of liquid developing an electrostatic charge pattern carried on an electrically insulating substrate to obtain a water-soluble dye image corresponding to the charge pattern, the improvement which comprises:

A. forming on a hydrophobic surface an image of electrostatically attractable toner particles corresponding to such charge pattern using a liquid developer comprising said toner particles admixed in a liquid carrier vehicle having a dielectric constant less than about 3.0 and a resistivity greater than about 10 ohm-cm, said toner particles comprising a binder and a colorant consisting essentially of an acidic, water-soluble dye, said binder comprising a hydrophobic copolymer having as a repeating unit thereof a group derived from an ethylenically unsaturated polymerizable monomer comprising two to about 19 carbon atoms and a basic nitrogencontaining moiety;

B. contacting said toner particle image carried on said hydrophobic surface with an activating solution comprising an acidic aqueous solution having a pH less than about 6.5 to convert said hydrophobic copolymer contained in said toner particles to the hydrophilic form thereof; and

C. contacting said hydrophilic dye-containing particle image carried on said hydrophobic surface with a receiving element comprising a hydrophilic colloid layer containing a mordant for the dye to transfer and mordant said image to the colloid layer of the receiving element.

8. The invention as described in claim 7 wherein (B) and (C) are performed substantially simultaneously by pre-wetting the receiving element with said activating solution and contacting the resultant wetted receiving element with the toner particle image formed in (A).

9. The invention as described in claim 7 wherein said ethylenically unsaturated polymerizable monomer is selected from the group consisting of aminostyrene, vinyl pyridine, vinyl amine, amino-substituted alkyl methacrylates, amino-substituted alkyl acrylates and vinyl imidazole.

10. The invention of claim 7 wherein a multiple-color image is obtained on said receiving element by successively repeating (A) to (C) to form superimposed, in register, on said receiving element a series of individual dye images, each of said images representing an individual color separation image of an original image.

11. The invention as described in claim 7 wherein said acidic water-soluble dye is selected from the group consisting of 4,4'-bis( l-acrylamino-3,6-disodiumsulfo- 8-hydroxynaphthalene-7-azo )-2 ,2 ,5 ,5 -tetramethyltriphenylmethane; 4,4 -bis(p-alkoxyphenylazo )-2 ,2 disodiumsulfostilbene; 4,4'-(6,8-disodiumsulfo-2- naphthylaminocarbonyl-p-phenylazo)-3 ,3 dimethylurea; l-( p-sulfophenyl-3-methyl-4-(4,8- disulfonaphthyl-Z-azo )-5-pyrazolone trisodium salt; (1- hydroxy-3,6-disodiumsulfo-8-acetylaminonaphthyl-2- azo )-4-( 1 -hydroxy-3 ,6,disodiumsulfo-8- aminonaphthyl-Z-azo)-2,5-dimethoxy benzene; l-(psulfophenyl-3 -phenyl-4-( 2 ,5 -dichloro-4-sulfophenyl-l azo)-5-pyrazolone disodium salt; 1-carboethoxy-3- methyl-6-( 4 -chloro-2 '-sulfoanilino )-2-oxo-2,3 -dihydrobenzanthrone, sodium salt; S,5'-ureylenebis[2-(2- amino-6-sulfo-8-hydroxy-l -naphthylazo )benzenesulfonic acid], tetrasodium salt, C.I. 25380; 3,3'[4,4- ureylenebis( 2-rnethylphenylazo)]bis( l ,S-benzenedisulfonic acid), tetrasodium salt, CI. 29025; {2-anilino-4-[4-(3-carboxyA-hydroxyphenylazo anilinol-s-triazin-Z-ylamino }-4-hydroxy3-[4-(8-hydroxy-3 ,o-disulfol naphthylazo)-2 -methoxy-3-methylphenylazo]-2,7- naphthalenedisulfonic acid, pentasodium salt, C. I. 34045; 6,6'-ureylenebis {2-[4-(2,4-dichloro-6-sulfophenylazo)-3- methylphenylazo]- 1 -naphthol-3-sulfonic acid}; and anhydro-3 ,6-bis(diethylarnino)9-(2 ,4-disulfophenyl)-' xanthylium hydroxide, sodium salt, C. l. 4Sl00. 

2. The invention of claim 1 wherein said receiving surface comprises a hydrophilic colloid layer containing mordant for said acidic water-soluble dye.
 3. The invention of claim 1 wherein (B) and (C) are performed substantially simultaneously by pre-wetting the receiving element with said activating solution and contacting the resultant wetted receiving element with the toner particle image formed in (A).
 4. The invention as described in claim 1 wherein said ethylenically unsaturated polymerizable monomers comprises two to about 19 carbon atoms and a basic nitrogen-containing moiety.
 5. The invention as described in claim 1 wherein said ethylenically unsaturated polymerizable monomer is selected from the group consisting of aminostyrene, vinyl pyridine, vinyl amine, amino-substituted alkyl methacrylates, amino-substituted alkyl acrylates and vinyl imidazole.
 6. The invention of claim 1 wherein a multiple-color image is obtained on said receiving element by successively repeating (A) to (C) to form superimposed, in register, on said receiving element a series of individual dye images, each of said images representing an individual color separation image of an original image.
 7. In a method of liquid developing an electrostatic charge pattern carried on an electrically insulating substrate to obtain a water-soluble dye image corresponding to the charge pattern, the improvement which comprises: A. forming on a hydrophobic surface an image of electrostatically attractable toner particles corresponding to such charge pattern using a liquid developer comprising said toner particles admixed in a liquid carrier vehicle having a dielectric constant less than about 3.0 and a resistivity greater than about 1010 ohm-cm, said toner particles comprising a binder and a colorant consisting essentially of an acidic, water-soluble dye, said binder comprising a hydrophobic copolymer having as a repeating unit thereof a group derived from an ethylenically unsaturated polymerizable monomer comprising two to about 19 carbon atoms and a basic nitrogen-containing moiety; B. contacting said toner particle image carried on said hydrophobic surface with an activating solution comprising an acidic aqueous solution having a pH less than about 6.5 to convert said hydrophobic copolymer contained in said toner particles to the hydrophilic form thereof; and C. contacting said hydrophilic dye-containing particle image carried on said hydrophobic surface with a receiving element comprising a hydrophilic colloid layer containing a mordant for the dye to transfer and mordant said image to the colloid layer of the receiving element.
 8. The invention as described in claim 7 wherein (B) and (C) are performed substantially simultaneously by pre-wetting the receiving element with said activating solution and contacting the resultant wetted receiving element with the toner particle image formed in (A).
 9. The invention as described in claim 7 wherein said ethylenically unsaturated polymerizable monomer is selected from the group consisting of aminostyrene, vinyl pyridine, vinyl amine, amino-substituted alkyl methacrylates, amino-substituted alkyl acrylates and vinyl imidazole.
 10. The invention of claim 7 wherein a multiple-color image is obtained on said receiving element by successively repeating (A) to (C) to form superimposed, in register, on said receiving element a series of individual dye images, each of said images representing an individual color separation image of an original image.
 11. The invention as described in claim 7 wherein said acidic water-soluble dye is selected from the group consisting of 4,4''-bis(1-acrylamino-3,6-disodiumsulfo-8-hydroxynaphthalene-7-azo)-2, 2'',5,5''-tetramethyltriphenylmethane; 4,4''-bis(p-alkoxyphenylazo)-2,2''-disodiumsulfostilbene; 4,4''-(6,8-disodiumsulfo-2-naphthylaminocarbonyl-p-phenylazo)-3,3'' -dimethylurea; 1-(p-sulfophenyl-3-methyl-4-(4,8-disulfonaphthyl-2-azo)-5-pyrazolone trisodium salt; (1-hydroxy-3,6-disodiumsulfo-8-acetylaminonaphthyl-2-azo)-4-(1-hydroxy -3,6,disodiumsulfo-8-aminonaphthyl-2-azo)-2,5-dimethoxy benzene; 1-(p-sulfophenyl-3-phenyl-4-(2,5-dichloro-4-sulfophenyl-1-azo)-5-pyrazolone disodium salt; 1-carboethoxy-3-methyl-6-(4''-chloro-2''-sulfoanilino)-2-oxo-2,3 -dihydrobenzanthrone, sodium salt; 5,5''-ureylenebis(2-(2-amino-6-sulfo-8-hydroxy-1-naphthylazo)benzenesulfonic acid), tetrasodium salt, C.I. 25380; 3,3''-(4,4''-ureylenebis(2-methylphenylazo))bis(1,5-benzenedisulfonic acid), tetrasodium salt, C.I. 29025; 5-(2-anilino-4-(4-(3-carboxy-4-hydroxyphenylazo)-anilino)-s-triazin-2 -ylamino)-4-hydroxy-3-(4-(8-hydroxy-3,6-disulfo-1-naphthylazo)-2-methoxy-3 -methylphenylazo)-2,7-naphthalenedisulfonic acid, pentasodium salt, C. I. 34045; 6,6''-ureylenebis(2-(4-(2,4-dichloro-6-sulfophenylazo)-3-methylphenylazo)-1 -naphthol-3-sulfonic acid); and anhydro-3,6-bis(diethylamino)-9-(2,4-disulfophenyl)-xanthylium hydroxide, sodium salt, C. I.
 45100. 